No one knows what it means, and its meaning is lost. I was browsing the Oxford English Dictionary and saw it in a list. Here’s the sum and whole of its entry:
"Obs. Origin and sense unknown."
Less than enlightening! It includes one original reference, in the diary of Simon Forman, which I find elsewhere referenced in The Notorious Astrological Physician of London.
"This yere I bought many pictures about our Lady dai. This sommer I had my own pictur drawn, and mad my purple gowne, my velvet cap my velvet cote my velvet breches my taffety cloke, my had & many other thinges & did let my hear & berd growe. . . . I bought my swachele sword this yer & did the hangers with siluer."
It’s theorized that swachele may mean “swash,” akin to flashy or a quality of swagger.